The present invention relates to permanent magnet synchronous machines and more particularly to a method and apparatus for determining the absolute position of a motor rotor without use of position or velocity feedback transducers.
Many motion control applications which utilize a motor require motor rotor position information so that the motor and hardware linked thereto can be precisely regulated. For example, in a package handling system a motor rotor may be linked to a piston cylinder for moving a package laterally from a first position on a first conveyor to a second position on a second conveyor. To move the package from the first position to the second position the rotor may be required to rotate precisely 4.5 times in a clockwise direction. Thereafter, to park the piston out of the way of another package on the first conveyor the rotor may be required to rotate precisely 4.5 times in a counter-clockwise direction. In this case initial rotor position is important as well as rotor position during piston movement among the first and second positions. Even a small rotor position error may cause incorrect and unacceptable package alignment with the second conveyor resulting in system or package damage.
While the present example illustrates how rotor position is important prior to movement and during relatively slow rotor rotation, rotor position during fast rotor rotation is also important. For example, most motor driven industrial machines require some type of motor speed control. To this end, a speed feedback loop is typically provided so that motor speed can be regulated as a function of the difference between actual rotor speed (i.e. the feedback speed) and a reference or commanded speed.
Moreover, in virtually all control schemes involving permanent magnet synchronous motors rotor position is required to facilitate commutation. To facilitate precise motor control both a controller and some form of rotor position sensor are typically required. One common position sensor is an encoder. An encoder usually detects some known rotor nuance and uses the detected nuance to determine position. The nuance may include a plurality of magnets or light reflectors which are equi-spaced about a rotor end surface. In these cases the sensor would be a magnetic or light sensor, respectively, which detects the nuances and determines rotor position therefrom.
The encoder feeds the rotor position back to a motor controller via a hardwire feedback loop. In addition to a position sensor, some systems also utilize a velocity transducer to dynamically predict rotor position during the periods between encoder sampling times to more precisely identify rotor position.
Unfortunately, position and velocity sensors require additional system hardware and therefore increase both system procurement (e.g. parts) and manufacturing (e.g. assembly) costs. In addition, because sensors are subject to malfunction, sensor repair and system downtime prior to sensor repair can increase operating costs appreciably.
For these reasons the industry has exerted a great deal of effort developing sensorless rotor position identifying techniques. These techniques can generally be grouped into two different categories including techniques in a first category which rely on processing back electromotive force (bemf) signals and a second category including techniques which rely on identifying rotor magnetic saliency nuances.
With respect to the first category, these techniques generally track bemf caused by a rotating motor to identify rotor position. Unfortunately, bemf techniques have several shortcomings. For example, the bemf signal is sensitive to parameter variations at virtually all speeds. In addition, at standstill (i.e. zero rotor velocity) there is no bemf signal and therefore position cannot be determined using typical bemf methods.
With respect to the second category including systems which use magnetic rotor saliency to determine rotor position, many different schemes have been developed for identifying rotor position, each of which suffers from one or more shortcoming. For example, the publication entitled "Sensorless Position Control Of Permanent Magnetic Drives" by C. French et al., which was published in Proc. IEEE-IAS Annual Meeting, 1995, pp. 61-68 describes one system which uses a flux linkage based approach which requires three phase current and three phase voltage measurements. Having to measure three phase currents and three phase voltages is hardware intensive and therefore is relatively expensive and undesirable. In addition, with this scheme position resolution is decreased when speed is reduced.
Another solution which relies on m,magnetic saliency is described in U.S. Pat. No. 5,117,165 which is entitled "Closed-Loop Control Of a Brushless DC Motor From Standstill To Medium Speed", issued to Cassat et al. on May 26, 1992. This patent teaches a sensorless method to roughly estimate rotor position primarily for the purpose of starting a motor in the correct direction. To this end, this patent teaches providing separate positive and negative current pulses to each of several different stator windings, determining phase fluxes for each winding phase, determining the polarity of flux differences between each unique pair of fluxes and using the difference polarities to determine general rotor position within .pi./m radians within one electrical period wherein m is the number or stator phases (i.e. windings).
While this solution is simple and relatively inexpensive and may be sufficiently accurate for some applications, this solution is clearly not accurate enough for other applications which require rotor position to be determined more precisely.
Therefore, it would be advantageous to have a method for determining rotor position of permanent magnet synchronous machines at standstill wherein the method is inexpensive, simple, accurate and relatively maintenance free.